The rate of nuclear fission reactions in nuclear reactors are a function of the number of neutrons available to carry on the neutron triggered chain reactions. The fission of a fuel nucleus releases one or more neutrons, and one neutron is required to sustain the chain reaction. Many of the design features of a nuclear reactor are based upon their impact on the neutron economy. In particular, materials for use in nuclear reactors are selected for their neutron capture cross-sections, .sigma., along with other properties. Low .sigma. materials are selected for most reactor components, such as support structure, fuel rod cladding, moderators, and the like. Whereas, high a materials are selected for control rods and burnable poison shims. The more transparent the internal materials of construction of a nuclear reactor are to such thermal neutrons, the more efficiently the reactor will function since a certain number of these neutrons are necessary to sustain the nuclear reaction. A "burnable poison shim" is a high .sigma. material added in a carefully selected quantity to decrease neutron flux early in a fuel cycle, and to become transparent or neutral after neutron adsorption so that late in a fuel cycle more of the fission neutrons are absorbed by fuel.
Gadolinium (Gd) is a rare earth element which finds its greatest potential application as a neutron absorber in a nuclear reactor. Gadolinium has historically been a material of construction for nuclear reactor control rods. In such applications, it serves its function by absorbing thermal neutrons having an average thermal neutron capture cross-section of about 49,000 barns (10.sup.-24 cm.sup.2). However, some isotopes of gadolinium are much more efficient at neutron capture because of their much greater thermal neutron capture cross-sections, .sigma..
Gadolinium is the element with the highest a for its natural isotope mixture. Only two isotopes of gadolinium, Gd.sup.155 and Gd.sup.157, are particularly attractive as having high neutron capture cross-sections as shown in the following Table 1 which is an approximation of the distribution of the naturally occurring gadolinium isotopes.
TABLE 1 ______________________________________ Naturally Occurring Gadolinium Isotopic Distribution Isotope Thermal Neutron Capture Mass No. Natural Atomic % Cross-Section .sigma. (Barns) ______________________________________ 152 0.20 1100 154 2.15 85 155 14.73 61,000 156 20.47 1.5 157 15.68 254,000 158 24.85 2.5 160 21.90 0.77 ______________________________________
As illustrated, Gd.sup.155 and Gd.sup.157 have the highest thermal neutron capture cross-sections .sigma. but comprise only about 30 percent of the natural gadolinium. There is a need, therefore, to separate the high neutron capture Gd isotopes from the other Gd isotopes and also from other rare earth elements. There is a need for a technique which could simultaneously isolate the high neutron cross-section Gd.sup.155 and Gd.sup.157 isotopes from a mixture of other Gd isotopes.
As shown by the separation schemes in FIG. 1, the separation of gadolinium from other lanthanides (i.e., La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) is a complex issue. Different chemistries are required for various fractionation operations of the individual lanthanides, since no two lanthanides have identical chemistries. The kinetics of separations also differ which implies a kinetically driven, rather than equilibrium driven, separation operation. In addition, the chemistries for performing isotopic enrichment, i.e., separation of isotopes and purification, from the individual fractions will also differ and require different isotope enrichment schemes. For example, as shown in FIG. 1, gadolinium separation proceeds better in nitrate systems while erbium separation proceeds better in phosphate and ferricyanide systems.
Much of the difference is due to the basic differences in the electronic configuration of various elements which manifest themselves in different hydrolytic and complex formation behavior and, accordingly, influence the separation behaviors of these elements, such as in ion exchange separations. The acidity of lanthanides will increase in the order of decreasing ionic radii as defined by the electronic configuration of the elements. Acidity is an important property that determines the ion exchange behavior; for example, as Gd hydrates in water it tends to displace a hydrogen ion from a water molecule of hydrolysis. Moreover, Gd is a trivalent species and will tend not to form stable complexes with anionic species in solution. Gd typically will go into solutions as a cation and stringent solution concentrations are required to form its anionic complexes.
Ion exchange separation schemes involve both complex formation and hydrolysis formation phenomena. In cation exchange, H.sup.+ is liberated and in anion exchange, OH.sup.- is liberated from the ion exchange media. The atomic radii of lanthanides increases with atomic number. The smaller radii-elements are typically absorbed as cations on cationic ion exchange resins preferentially, and the smaller radii elements are then eluted from the cationic ion exchange resin with a solution containing a strongly complexing anion, such as citric acid. It is expected that individual fractionation and isotopic separation will occur differently with different elements.
Prior efforts at isotope separation have involved some type of solvent extraction. But these separation techniques are generally only able to separate only one isotope at a time, and, therefore, require a plurality of extraction trains and purification steps. Other efforts for simultaneous separation of isotopes other than gadolinium using ion exchange in a continuous annular chromatograph are taught in U.S. Pat. Nos. 5,098,678 (Lee, et al.), 5,110,566 (Snyder, et al.) and 5,174,971 (Snyder, et al.).
Recent efforts of separating gadolinium from its isotopes and also from other rare earth elements have been focused on ion exchange chromatography. U.S. Pat. No. 4,711,768 (Peterson, et al.) teaches a method of separating gadolinium isotopes, i.e., Gd.sup.155, Gd.sup.156 and/or Gd.sup.157, from mixtures containing the same by a batchwise chromatographic process using a plurality of liquid chromatographic columns, an ion exchange resin stationary phase and an eluant solution mobile phase, and sequentially separating the isotopes one at a time.
It is difficult to effect a good separation using traditional chromatography due to problems associated with controlling the elution of isotope product peaks. Consequently, it is difficult to get the concentration of gadolinium isotope in the product of elution as high as desired. Thus, larger than desired volumes of spent eluant must be used which poses a waste management problem. In addition, this batchwise approach is both expensive to build and difficult to control. There is a need to concurrently or simultaneously separate the high thermal neutron capture cross-section Gd isotopes, especially Gd.sup.155 and Gd.sup.157, in a continuously operating ion exchange chromatographic separation technique while also using reduced elution volumes.